This invention relates to a button cell containing at least one positive electrode, one negative electrode and an interposed separator arranged one on top of the other in a flat gas-tight housing, and also an electrolyte.
Button cells conventionally comprise a housing cup and a lid which are both produced as punched and drawn parts from nickel-plated deep-drawn strip steel. Normally, the housing cup is positive in polarity and the lid is negative in polarity. Button cells of this construction may contain different electrochemical systems. Button cells with rechargeable Ni/Cd systems are very widespread. Recently, button cells with metal oxide/metal hydride systems, in particular Ni/Nickel hydride cells, have been acquiring increasing importance. In addition to these secondary systems, which also include secondary lithium systems, primary lithium systems and other high-energy systems such as silver oxide/zinc systems, are available in button-cell form.
At least the positive electrodes of many conventional button cells, and for alkaline Ni/Cd systems, also the negative electrodes, are mass electrodes in the form of pressed powder pellets enclosed in a fine-mesh pure-nickel wire web. A fine-pore separator composed of a plastic mat is arranged between the electrodes. A corrugated spring contact on the back of the negative electrode produces a defined compressive force on the plate group and consequently results in a uniform contact resistance between the electrodes and the housing.
The cells may be closed by flanging the rim of the cup over the lid. In this arrangement, a plastic ring is used, preferably simultaneously serving as a sealing ring and to insulate the cup from the lid.
The manufacture of pressed powder electrodes for button cells is laborious and generally unsuitable for an efficient production cycle, which is largely determined by automatic operations. The powder mass is first dispensed and pressed into a pellet. The pellet is then placed in an enclosure composed of a nickel wire web and covered with a small plate composed of the same web material. The enclosure is completed by wrapping a rim projecting beyond the pellet over the small cover plate. The electrode is then compressed (once again) by a pressure ram.
Because of the discontinuous sequence of these operations, this method is not only time-consuming, but also expensive due to the cost of the nickel wire netting, and because the nickel wire netting substantially exceeds the amount necessary for electrode reinforcement and for current collection.